A proprietary format is a file format of a company, organization, or individual

• that contains data that is ordered and stored according to a particular encoding-scheme, designed by the company or organization to be secret, such that the decoding and interpretation of this stored data is only easily accomplished with particular software or hardware that the company itself has developed. The specification of the data encoding format is not released, or underlies non-disclosure agreements.

Or a proprietary format is a file format

• whose encoding is in fact published, but is restricted (through licences) such that only the company itself or licencees may use it.

In contrast an open format is a file format that is published and free to be used by everybody.